Tree mixers, also known as analog multipliers or Gilbert multipliers, have been widely used in integrated circuits for communications systems for many years. As is known for example from B. Gilbert, "A Precise Four-Quadrant Multiplier with Subnanosecond Response", IEEE Journal of Solid-State Circuits, Vol. SC-3. Pages 365-373, December 1968, such a mixer or multiplier typically comprises a first or lower differential pair of common emitter transistors to the bases of which a first differential analog input signal is supplied, and two second or upper differential pairs of transistors whose bases are supplied with a second differential analog input signal and whose collector-emitter paths conduct the currents of the lower pair of transistors to produce in their collector circuits an analog output signal which represents the product of the input signals. A single current source in the emitter circuit of the lower pair of transistors provides bias current to all six transistors. For use as a mixer in a radio communications receiver or transmitter, for example an input signal is applied to the lower pair of transistors and a local oscillator signal is applied to the two upper pairs, or upper quad, of transistors.
Degeneration resistors connected to the emitters of the lower differential pair of transistors serve to linearize the input stage to accommodate larger input signals without distorting.
Such a circuit provides advantages of good rejection of the input signals at the output, good power supply rejection, and the possibility of conversion gain.
Attempts to enhance the performance of tree mixers have been proposed. For example, two class AB biasing schemes have been proposed by J. Durec et al in "Motorola's Mosaic V Silicon Bipolar RF Building Blocks Fill Gaps in High Performance Low Power Wireless Chip Sets", Proceedings of the 4th Wireless Symposium, Santa Clara, U.S.A., pages 218-223, 1996 and by B. Gilbert in "Design Considerations for BJT Active Mixers. (course notes)" 1995. This technique enables the reduction in the DC bias in the mixer, thereby reducing shot noise in the mixing quad transistors, without degrading linearity. However, the bias schemes employed may negatively impact overall noise performance, and distortion characteristics may vary significantly over a wide range of signal inputs. In addition, the class AB scheme in these mixers precludes the application of a differential input signal to the mixer.
A further example of a tree mixer which is an improvement over the basic tree mixer is described in U.S. Pat. No. 5,532,637 which issued on Jul. 2, 1996 to Khoury et al. This scheme also improves noise performance by reducing the DC current flowing in the upper quad thereby reducing the shot noise in the upper quad transistors. While this circuit maintains good linearity for small signal inputs, it ultimately reduces the maximum undistorted signal that can be obtained at the output unless complicated control circuitry is employed.
An object of the invention is to provide a tree mixer which obviates or mitigates one or more of the above described disadvantages.